Method and device for treating exhaust gas

ABSTRACT

A process and an apparatus for treating an exhaust gas, in which a raw gas and high-boiling intermediate products contained in the exhaust gas let out from a CVD system employing a silicon-containing gas is brought into contact with a transition metal such as nickel or a silicide of such transition metals to decompose or convert them into stable halides, followed by detoxication treatment of the harmful components contained in the exhaust gas.

TECHNICAL FIELD

The present invention relates to a process and an apparatus for treatingan exhaust gas, more particularly to a process and an apparatus fordetoxicating an exhaust gas let out from a chemical vapor deposition(CVD) system for forming silicon epitaxial films, polycrystalline filmsor amorphous films using silicon-containing gases (silane halide gases)in a semiconductor manufacturing process. The silicon-containing gasesrefer to halogenosilane gases such as trichlorosilane (TCS: SiHCl₃) anddichlorosilane (DCS: SiH₂Cl₂), and silicon halide gases such as silicontetrachloride.

BACKGROUND ART

The epitaxial (single crystal growth) process, which is a silicon CVDprocess, is employed for preparation of substrates for field-effect MOS(metal-oxide-semiconductor) transistors or for formation of emitterlayers in bipolar transistors. The epitaxial process is generallycarried out by using as a raw gas a silicon-containing gas such as TCSand DCS which is diluted with hydrogen before introduction into aprocess chamber and by heat-decomposing the raw gas by heating thesubstrate placed in the process chamber to about 1100° C. to effectdeposition of silicon on the substrate. The above process is generallycarried out under a pressure conditions of atmospheric to 100 Pa(Pascal).

Meanwhile, the polycrystal growth process is employed for forming gateelectrodes of field effect MOS (metal oxide semiconductor) transistorand ground layers for capacitors. In the polycrystal growth process, asilicon-containing gas such as TCS and DCS is diluted with hydrogen, andthe thus diluted gas is introduced into a process chamber in which asubstrate heated to about 800° C. is loaded to effect heat decompositionof the raw gas to achieve deposition of silicon on the substrate. Thisprocessing is usually carried out under a vacuum condition of about 100Pa.

Further, in such processes, for the purpose of control of moisture to beadsorbed by wafers as they are loaded in and out of the process chamber,a moisture monitor (an optical analyzer for optical measurement; e.g.,Fourier Transform infrared (FTIR) spectrophotometer) is occasionallyattached to an exhaust piping system.

In such crystal growth process as described above, the amount of the rawgas which is introduced into the process chamber for the purpose ofsilicon deposition and which contributes actually to the deposition ofsilicon on the substrate is about 5%, and the most of the rest of theraw gas is exhausted without contribution together with intermediateproducts (about several % of the total amount) from the chamber. Theexhaust gas let out from the chamber is detoxicated by a detoxicatingunit which removes the raw gas and intermediate products, and onlyhydrogen as the carrier gas and nitrogen as the purge gas are releasedinto the atmosphere.

In the epitaxial process, however, there is a problem that intermediateproducts (by-products) formed during the process adhere or deposit onthe inner wall surface of the exhaust piping to be likely to causeclogging of the exhaust piping with the deposit. Further, lighttransmitting windows of optical analyzers are tarnished with the depositto make it sometimes difficult to carry out accurate measurement. Suchintermediate products include compounds of silicon and chlorine or ofsilicon and hydrogen, and these compounds form polymers at roomtemperatures on the inner wall surface of the exhaust piping. Thepolymers formed are converted to highly reactive (self ignitable orexplosive) materials, for example, polysiloxanes, by the moisturecontained in the atmosphere. Accordingly, when the exhaust piping isdisassembled to be open to the atmosphere in order to remove thepolymers deposited on the inner wall surface of the exhaust piping,various preparations and contrivances are required, being causative ofdropping the operation efficiency of the CVD system.

Meanwhile, it is practiced to feed an etching gas such as chlorinetrifluoride (ClF₃) into the exhaust piping in order to preventdeposition of the polymers. It is true, however, that the intermediateproducts deposited on the inner wall surface of the piping can beremoved according to this method, but the method involves a problem inthat the exhaust piping itself is corroded by the strong etchingproperty of the etching gas or the etching gas can even cause formationof holes in the piping.

Further, the etching gas such as ClF₃ and the raw gas employed in theepitaxial process cannot usually be treated by a single detoxicatingunit, so that a plurality of detoxicating units must be used selectivelydepending on which gas is fed.

On the other hand, there is proposed a method in order to preventintermediate products from adhering or depositing on the exhaust pipingto heat the piping constantly to a temperature of about 150° C.According to this method, however, if the temperature of the piping islow at some parts, the intermediate products deposit selectively to suchlow-temperature parts. The piping between the detoxicating unit and theCVD system usually contains complicated bends from the requirement ofreducing the installation area, and it is difficult to heat orheat-insulate the piping uniformly. Actually, maintenance of the pipinghas been carried out by disassembling the piping to remove theintermediate products deposited at the low-temperature portions.

While a scrubber employing water is frequently used for detoxication ofTCS or DCS, solid silicon dioxide (SiO₂) is formed by the reactionbetween water and TCS or DCS, so that the circulation water employed inthe scrubber is provided with means for removing SiO₂. However, sincethe thus removed SiO₂ contains hydrogen, it cannot be exhausted as such.Thus, it has been practiced to carry out treatment of SiO₂ by reactingit with hydrogen fluoride (HF). Since these procedures are carried outas periodical maintenance of the detoxicating unit, not only theoperation rate of the CVD system is lowered, but also chemical agentsfor removing the SiO₂ formed, personnel, etc. cost additionally.

DISCLOSURE OF THE INVENTION

It is an objective of the present invention to provide a process and anapparatus for treating an exhaust gas, which can reduce or eliminateperiodical maintenance of the exhaust piping and detoxicating units byconverting the raw gas employed in the crystal growth process or tohighly volatile halides and exhausting the thus obtained halides to thedetoxicating unit or a recovery unit without causing adhesion ordeposition of the intermediate products in the exhaust piping system.

In order to attain the above objective, in the process for treating anexhaust gas let out from a CVD system for forming a silicon film using asilicon-containing gas according to the present invention, an unreactedraw gas and an intermediate product contained in the exhaust gas aresubjected to a decomposition or conversion reaction treatment, and thenharmful components contained in the exhaust gas are detoxicated. Thedecomposition or conversion reaction treatment is carried out bybringing the exhaust gas into contact with a transition metal or atransition metal silicide heated to 400° C. or higher. Further, thedecomposition or conversion reaction treatment is carried out afteraddition of hydrogen gas to the exhaust gas.

The apparatus for treating an exhaust gas let out from a CVD system forforming a silicon film using a silicon-containing gas according to thepresent invention is provided with decomposition reaction means forcarrying out decomposition or conversion reaction of an unreacted rawgas and an intermediate product contained in the exhaust gas;detoxicating means for detoxicating harmful components contained in theexhaust gas let out from the decomposition reaction means; and means forheating or maintaining an exhaust gas passage from the CVD system to thedecomposition reaction means to or at a predetermined temperature. Theexhaust gas passage is provided with hydrogen gas adding means foradding hydrogen gas to the exhaust gas. The decomposition reaction meansis provided with a reactor packed with a transition metal or atransition metal silicide and means for heating the transition metal ortransition metal silicide to a predetermined temperature.

According to the present invention, since the unreacted raw gas and thelike can be decomposed or converted into hydrogen chloride (HCl) whichcan be treated easily, no deposit is formed on the inner wall surface ofthe piping, and further the periodical maintenance of removing depositbecomes unnecessary, improving operation rate of the CVD system.Further, since no SiO₂ is formed during the detoxication treatment, theload to be applied to the unit in the detoxicating treatment is reducedand the mechanism for removing SiO₂ having been installed conventionallybecomes unnecessary, resulting in curtailment of the cost of thedetoxication treating unit. Further, cleaning of the exhaust pipingusing ClF₃ becomes unnecessary, and thus the cost required for thecleaning can be reduced, and also the exhaust piping can be simplified.In addition, no damping of light occurs when monitored using an opticalmeasuring unit installed in the exhaust piping, so that accuratemeasurement can be performed and that maintenance of the optical windowsin measuring instruments can be eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram showing an embodiment where the apparatus fortreating an exhaust gas according to the present invention is applied toa CVD system;

FIG. 2 is a chart showing results of gas component measurement in TestExample 1;

FIG. 3 is a chart showing results of gas component measurement in TestExample 2;

FIG. 4 is a chart showing results of gas component measurement in TestExample 3;

FIG. 5 is a chart showing results of gas component measurement in TestExample 4;

FIG. 6 is a chart showing results of gas component measurement in TestExample 5; and

FIG. 7 is a chart showing results of gas component measurement in TestExample 6.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a system diagram showing an embodiment where the apparatus fortreating an exhaust gas according to the present invention is applied toa CVD system. This CVD system, which is a so-calledsingle-wafer-processing resistance heating silicon epitaxial system, isequipped with a process chamber 12 for loading substrates (wafers) 11, araw gas source 13 for supplying a raw gas, or a cleaning gas into thechamber 12, an atmospheric gas source 14 for supplying an atmosphericgas for maintaining a predetermined atmosphere in the process chamber12, a purge gas source for supplying a purge gas into the chamber 12, agas supplying system 16 for controlling flow rates of these gases andthe like, and pumps for exhausting the gas from the process chamber 12(a turbo-molecular pump 17 and a dry pump 18). The atmospheric gas isthe same as the carrier gas for carrying the raw gas.

The process chamber 12 is juxtaposed to a loading chamber (not shown)via a gate valve 19. The process chamber 12 is provided with a heatingequipment (not shown) for heating a susceptor 20 for loading the wafer11 and the wafer 11 itself to a predetermined temperature.

In an exhaust system piping 21 including the pumps 17 and 18, a reactor22 and a detoxicating unit 23 are juxtaposed. The former is means forcarrying out decomposition or conversion reaction of the unreacted rawgas and high-boiling intermediate products contained in the exhaust gas,and the latter is means for carrying out detoxication treatment ofharmful components contained in the exhaust gas.

This CVD system carries out epitaxial treatment of wafers, while theprocess chamber 12 is maintained to have an internal pressure of 1 atmunder flow of raw gas diluted with a diluent gas and under exhaustion.To describe, for example, a typical sequence for carrying out p-typeepitaxial growth, a wafer 11 is introduced through the gate valve 19into the process chamber 12 to be loaded on the susceptor 20 under flowof a purge gas nitrogen at a flow rate of 2 l/min. After the gate valve19 is closed, the feed gas is switched from the nitrogen gas to hydrogengas (atmospheric gas) of 15 l/min to provide a hydrogen atmosphere inthe process chamber 12, and also the wafer 11 is heated to 1200° C.

After a wafer planarization treatment at 1200° C. in an atmosphere of 1atm hydrogen atmosphere for 30 seconds, the wafer heating temperaturewas lowered to 1150° C. and supply of an epitaxial reaction gas underthe 1 atm condition was started to carry out a treatment for 90 seconds.As the epitaxial reaction gas, a mixed gas of a gas containing 15 g/minof TCS in 7 l/min of hydrogen and a gas containing 150 cc/min ofdiborane in 14.6 l/min of hydrogen is used. Incidentally, when DCS isused in place of TCS, DCS is supplied at a rate of 10 g/min, while thewafer heating temperature is changed to 1080° C. Further, when an n-typeepitaxial growth layer is to be formed in place of p-type epitaxialgrowth layer, phosphine is supplied in place of diborane (at the sameflow rate as that of diborane).

After completion of the treatment, the feed gas is switched from theepitaxial reaction gas to 10 l/min of nitrogen gas, and the treatedwafer is unloaded. Next, the feed gas is switched to a mixed gas ofnitrogen gas and hydrogen chloride gas. While the mixed gas is suppliedat a rate of 7 to 15 l/min to maintain the mixed gas atmosphere in theprocessing chamber, the matters adhered or deposited in the processchamber are removed with the temperature and pressure being maintainedat 1150° C. and 1 atm respectively.

Finally, the feed gas was switched again to 10 l/min of nitrogen gas,and the internal temperature of the process chamber is lowered to aroundroom temperature. Thus, a cycle of treatment process is completed andreturns to the first step of wafer loading.

According to the sequence as described above, gases of variouscomponents are exhausted depending on the step to the exhaust piping 21to flow into the reactor 22. While the reactor 22 is to carry outdecomposition or conversion reaction of the unreacted raw gas andintermediate products contained in the exhaust gas and can treat themsuitably depending on the components of the gas to be treated, it ispreferred that the reactor 22 is packed with a transition metal catalystsuch as of iron (Fe), nickel (Ni), platinum (Pt), palladium (Pd),titanium (Ti), tungsten (W), tantalum (Ta), copper (Cu) or a silicide ofsuch transition metals and that the metal is heated to 400° C. orhigher.

As a heating equipment 24 for heating the catalyst, any heater such asan electric heater can be used. The heating temperature to be providedby this heating equipment 24, which may depend on the subject componentor the catalyst employed, is usually 400° C. or higher, for example, 400to 500° C., suitably. Meanwhile, the catalyst can be heated to thecritical temperature which depends on the material constituting thereactor 22, heat resistance of the catalyst, etc. However, there isobtained small effect of improving treating efficiency even if thecatalyst is heated to such high temperatures unnecessarily, leadingmerely to loss of energy. Meanwhile, some catalysts can causedissociation of HCl formed to generate active hydrogen radical andaccelerate embrittlement of the material constituting the reactor 22,when they are heated to 500° C. or higher.

Further, in view of maintenance (replacement or activation of catalysts)etc. of the reactor 22, it is desirable to install a plurality ofreactors 22 in parallel and to be used switchably. It is also preferredthat the discharge piping up to the reactor 22, i.e. an exhaust gaspassage 25 from the outlet of the process chamber 12 and through thepumps 17 and 18 to the reactor 22, is provided with a heating equipment26 and the like and is heated to a suitable temperature, for exampleabout 150° C. so as to prevent deposition from occurring in the passage.However, since the gas temperature in the process chamber 12 is high,the passage 25 need not be wound with a high-capacity heater, but it issometimes good enough to wind the passage 25 with a heat-insulatingmaterial and keep the temperature of the passage 25 by it. Accordingly,a suitable heating or heat-insulating equipment may be used depending onthe length, material, etc. of the exhaust gas passage, and suchequipment can be omitted in the case where the exhaust gas passage isshort to allow flowing of a gas having a sufficient temperature into thereactor 22.

When an exhaust gas from a CVD system is introduced to the reactor 22having such constitution, TCS, DCS and by-products (Si_(x)Cl_(y)) per seare reacted to be decomposed or converted to other substances. Thegreatest part of chlorine (Cl) is converted to HCl, whereas Si is bondedto the transition metal catalyst or forms a highly volatile halide, suchas silicon tetrachloride (SiCl₄). Likewise, boron (B) and phosphorus (P)in diborane and phosphine are removed by bonding to the catalyst.

While hydrogen is necessary in such reactions, there is no inconveniencefor the reactions to take place, since TCS and DCS per se containhydrogen and since hydrogen is used as the atmospheric gas or diluentgas in the usual epitaxial treatments, facilitating decomposition intoHCl. However, in the cases where there is a lack of hydrogen in theexhaust gas, for example, when plasma is used for assisting the growthreaction, a hydrogen gas adding equipment (passage) 27 may be attachedto an exhaust gas passage 25 on the upstream side of the reactor 22 tosupply an adequate amount of hydrogen to it. Particularly, decompositionor conversion of SiCl₄ formed can fully be carried out by maintainingthe amount of hydrogen suitably, allowing only HCl, which can be treatedextremely easily, to be contained as the toxic component in the gasflowing out of the reactor 22. Further, similar effects can be exhibitedeven when the reactor 22 is interposed between the process chamber 12and the pump 17, or between the pump 17 and the pump 18.

As described above, since the decomposition or conversion reaction ofthe unreacted raw gas and by-products contained in the exhaust gascarried out in the reactor 22 eliminates presence of TCS, DCS,by-products, etc. on the downstream side infra of the reactor 22, thereoccurs neither adhesion nor deposition of such gas components as far asthe detoxicating unit 23, and HCl and SiCl₄ formed in the reactor 22 caneasily be removed from the exhaust gas by absorption in water in thedetoxicating unit 23. Further, HCl can be recovered by adding an HClrecovering function to the detoxicating unit 23 to reutilize it in thestep of removing the matters adhering or deposited in the processchamber and the like.

TEST EXAMPLE 1

In a system of the constitution as illustrated in FIG. 1, an FTIR wasattached to the downstream side of the reactor 22 packed with a nickelcatalyst, and components of the gas flowing out of the reactor 22 wereanalyzed. While film-forming treatment was carried out under feeding ofa sample nitrogen gas containing 2300 ppm of TCS to the CVD system, theheating temperature of the reactor was elevated with time to measurechanges in the components in the gas flowing out of the reactor. Theresults are shown in FIG. 2.

As is clear from FIG. 2, TCS was decomposed completely at 400° C. toform HCl and SiCl₄. Meanwhile, no by-product (SiCl₂) having lowvolatility and high reactivity was detected.

TEST EXAMPLE 2

In the system as shown in FIG. 1, the procedures of Test Example 1 wererepeated analogously, except that the sample gas was replaced by anitrogen gas containing 2000 ppm of DCS. Consequently, DCS wasdecomposed fully at about 150° C. and TCS formed from DCS was alsodecomposed at about 360° C., as shown in FIG. 3. What were detected at400° C. were HCl and SiCl₄. Here again, no by-product was detected.

TEST EXAMPLE 3

In the system as shown in FIG. 1, the procedures of Test Example 1 wererepeated analogously, except that hydrogen gas was added to the samplegas used in Test Example 1. The results are shown in FIG. 4. It can beunderstood from the results that TCS decomposes fully at about 350° C.and that SiCl₄ also decomposes at about 400° C. to form HCl only. Hereagain, no by-product was detected.

TEST EXAMPLE 4

In the system as shown in FIG. 1, the procedures of Test Example 2 wererepeated analogously, except that hydrogen gas was added to the samplegas used in Test Example 2. Consequently, as shown in FIG. 5, it can beunderstood that DCS decomposes fully at about 150° C. and TCS formedfrom DCS decomposes at about 360° C., that SiCl₄ formed also decomposesat about 400° C., and that what is formed finally as a residue is HClonly. Here again, no by-product was detected.

TEST EXAMPLE 5

In the system as shown in FIG. 1, the procedures of Test Example 3 wererepeated analogously, except that the nickel packed into the reactor 22was replaced with nickel silicide. Consequently, TCS and SiCl₄ weredecomposed fully at about 350° C. to form HCl only, as shown in FIG. 6.Here again, no by-product was detected.

TEST EXAMPLE 6

In the system as shown in FIG. 1, the procedures of Test Example 1 wererepeated analogously, except that an argon gas containing 250 ppm of TCSwas supplied to the reactor 22 packed with an iron catalyst. It shouldbe noted here that the upper limit temperature of the reactor was 600°C. The results are as shown in FIG. 7. As shown clearly in FIG. 7, TCSstarted decomposing from at about 300° C. and decomposed fully at about500° C. No DCS was formed, and only HCl was detected.

In each of the above test examples, the amount of light transmittedthrough the light transmission window of the FTIR unit was measured, andthere was observed no drop in the amount of light transmission at all.

What is claimed is:
 1. A process for treating an exhaust gas let outfrom a CVD system for forming a silicon film using trichlorosilane anddichlorosilane, the process comprising: subjecting unreactedtrichlorosilane and dichlorosilane and an intermediate product containedin the exhaust gas to a decomposition treatment, the decomposition iscarried out by bringing the exhaust gas into contact with heatedtransition metal or a transition metal silicide to form hydrogenchloride; and detoxicating harmful components contained in the exhaustgas after the decomposition.
 2. The process for treating an exhaust gasaccording to claim 1, wherein the transition metal or the transitionmetal silicide is heated to 400° C. or higher.
 3. The process fortreating an exhaust gas according to claims 1 or 3, wherein thedecomposition treatment is carried out after addition of hydrogen gas tothe exhaust gas.
 4. An apparatus for treating an exhaust gas let outfrom a CVD system for forming a silicon film using trichlorosilane anddichlorosilane, the apparatus comprising: decomposition reaction meansfor carrying out a decomposition reaction of unreacted trichlorosilaneand dichlorosilane and an intermediate product contained in the exhaustgas into hydrogen chloride, the decomposition means is provided with areactor packed with a transition metal or a transition metal silicideand means for heating the transition metal or transition metal silicideto a predetermined temperature; detoxicating means for removing hydrogenchloride contained in the exhaust gas let out from the decompositionreaction means; and means for heating or maintaining an exhaust gaspassage from the CVD system to the decomposition reaction means to or ata predetermined temperature.
 5. The apparatus for treating an exhaustgas according to claim 4, wherein the exhaust gas passage is providedwith hydrogen gas adding means for adding hydrogen gas to the exhaustgas.